The specific aim of the proposed study is to utilize innovative translational research methods to examine the association between fear physiology, molecular genetics and posttraumatic stress symptoms (PTSS) using a gene-environment interaction approach. Inhibition of fear has been conceptualized as an intermediate neurobiological phenotype of posttraumatic stress disorder (PTSD), commonly viewed as a disorder of fear. PTSD is an ideal Candidate for a gene-environment interaction approach; however, existing molecular genetics research is hampered by the limitation of gene-environment correlation (which recognizes the fact that trauma exposure is in part determined by heritable factors). The proposed study utilizes a unique cohort of females enrolled in a longitudinal study at the time of a mass shooting at Northern Illinois University on February 14, 2008. The fateful nature of this trauma exposure minimizes the problem of gene-environment correlation. The proposed study builds upon the extensive trauma and mental health history available prior to the mass shooting and predicts that fear physiology and the pituitary adenylate cyclase-activating polypeptide (PACAP) will mediate the relationship between a fateful, shared trauma and PTSS. Recent research (Ressler et al., 2011) has reported a link between PACAP and PTSD symptoms in females but not males. For example, females demonstrated an association between PACAP38 blood levels and significantly increased startle reflexes to both the danger cue (CS+) and the safety cue (CS-), while the ADCYAP1R1 receptor SNP rs2267735 demonstrated significant association with PTSD and fear conditioning in females, but not males. Utilizing a subset (proposed N = 150) of this unique cohort exposed to a mass shooting, it is hypothesized that current fear physiology (e.g., laboratory fear potentiated startl to a fear conditioned cue, fear discrimination and fear extinction, as well as dark enhanced startle), combined with genetic and peripheral blood level markers (e.g., PACAP), will predict differential risk for PTSS as assessed from pre- to post-shooting (approximately 27 days post-shooting at Time 2), particularly among females who reported a more extensive trauma history prior to the mass shooting. The location of SNP rs2267735 within an estrogen response element holds promise in terms of explanatory power for sex differences in PTSD. To examine the impact of estrogen on PACAP-PAC1 gene expression, it is hypothesized that ADCYAP1R1 methylation levels will be most strongly related to PTSS among individuals with higher, as opposed to lower, levels of peripheral blood levels of estrogen. It is anticipated that findings wil inform understanding of the link between molecular genetics and the risk for PTSD, particularly with regard to sex differences in PTSD, ultimately leading to better tailoring of treatment methods for PTSD. PUBLIC HEALTH RELEVANCE: The proposed research aims to advance understanding of the underlying causes of posttraumatic stress disorder (PTSD), which is a recognized public health problem with significant societal and individual costs. The proposed research will explain the role that exaggerated physiological reactivity to startling stimuli, in combination with peripheral blood level markers of pituitary adenylate cyclase- activating polypeptide and estrogen, genetic factors, and past trauma history, plays in predicting posttraumatic stress following a campus shooting. This research will broaden understanding of the causes of PTSD and will shed light in particular on the nature of women's greater risk for PTSD.